A MANUAL OF PHYSIOLOGY 



percentage of iodine necessary, if the normal histological struc- 

 ture is to be maintained, is quite constant for a given species. 



So also the highest per- 

 centage of iodine asso- 

 ciated with any degree of 

 active hyperplasia (de-- 

 veloping goitre) is always 

 below the normal mini- 

 mum, as shown by Marine 

 in the dog, sheep, man, 

 and other mammals. As 

 active hyperplasia of the 

 thyroid (goitre) (Fig. 188) 

 develops, the iodine con- 

 tent of the gland, both 

 relative and absolute, de- 

 creases, until in extreme 

 degrees of the condition 



FIG. 189. MICROPHOTOGRAPH OF A COLLOID there may be no demon- 

 GLAND (GOITRE) (MARINE). 



The effect of administration of iodine is 

 shown in the return towards the normal 

 structure from a preceding active hyperplasia, 



such as is shown in Fig. 188. 



strable iodine present at 



all. Since the iodine is 



n 

 in 



_ 



as an iodine-protein com- 

 pound, the generalization 

 may be made that in the thyroid the iodine varies directly 

 with the amount of colloid, and inversely with the degree of 



hyperplasia. The ad- 

 ministration of any iodine- 

 containing substance to 

 animals with actively hy- 

 perplastic thyroids (goitres) 

 quickly (in two to three 

 weeks in dogs) induces a 

 histological change, the 

 end stage of which is the 

 so-called colloid goitre (Fig. 

 189). This is a reversion 

 to the normal histological 

 structure (Fig. 190), so far 

 as this is possible in a gland 

 which has once undergone 

 hyperplasia. The physio- 



on the thyroid may be 

 summed up as follows : Iodine is absolutely essential for the 

 normal activity of the gland. It prevents spontaneous hyper- 



